Coal tar is a primary by-product material produced during the destructive distillation or carbonization of coal into coke. While the coke product is utilized as a fuel and reagent source in the steel industry, the coal tar material is distilled into a series of fractions, each of which are commercially viable products in their own right. A significant portion of the distilled coal tar material is the pitch residue. This material is utilized in the production of anodes for aluminum smelting, as well as electrodes for electric arc furnaces used in the steel industry. In evaluating the qualitative characteristics of the pitch material, the prior art has been primarily focused on the ability of the coal tar pitch material to provide a suitable binder used in the anode and electrode production processes. Various characteristics such as softening point, specific gravity, percentage of material insoluble in quinoline, also known as QI, and coking value have all served to characterize coal tar pitches for applicability in these various manufacturing processes and industries.
Softening point is the basic measurement utilized to determine the distillation process end point in coal tar pitch production and to establish the mixing, forming or impregnating temperatures in carbon product production. All softening points referred to herein are taken according to the Mettler method or ASTM Standard D3104. Additional characteristics described herein include QI, which is utilized to determine the quantity of solid and high molecular weight material in the pitch. QI may also be referred to as α-resin and the standard test methodology used to determine the QI as a weight percentage include either ASTM Standard D4746 or ASTM Standard D2318. Percentage of material insoluble in toluene, or TI, will also be referred to herein, and is determined through ASTM Standard D4072 or D4312.
Mirtchi and Noel, in a paper presented at Carbon '94 at Granada, Spain, entitled “Polycyclic Aromatic Hydrocarbons in Pitches Used in the Aluminum Industry,” described and categorized the PAH content of coal tar pitches. These materials were classified according to their carcinogenic or mutagenic effect on living organisms. The paper identified 14 PAH materials which are considered by the United States Environmental Protection Agency to be potentially harmful to public health. Each of the 14 materials is assigned a relative ranking of carcinogenic potency which is based on a standard arbitrary assignment of a factor of 1 to benzo(a) pyrene or B(a)P. Estimations of potential toxicity of a pitch material may be made by converting its total PAH content into a B(a)P equivalent which eliminates the necessity of referring to each of the 14 materials individually, providing a useful shorthand for the evaluation of a material's toxicity.
A typical coal tar binder pitch is characterized as shown in Table I.
TABLE I Softening Point, ° C.111.3Toluene Insolubles, wt. %28.1Quinoline Insolubles, wt. %11.9Coking Value, Modified Conradson, wt. %55.7Ash, wt. %0.21Specific Gravity, 25/25° C.1.33Sulfur, wt. %0.6B(a)P Equivalent, ppm27,500
Two shortcomings with respect to the use of coal tar pitch in general, and more specifically in the aluminum industry, have recently emerged. The first is a heightened sensitivity to the environmental impact of this material and its utilization in aluminum smelting anodes. The other is a declining supply of crude coal tar from the coke-making process. Significant reductions in coke consumption, based upon a variety of factors, has reduced the availability of crude coal tar. This reduction in production of these raw materials is expected to escalate in the near future and alternative sources and substitute products have been sought for some period. No commercially attractive substitute for coal tar pitch in the aluminum industry has been developed, however.
There are two common methods of distilling coal tar, continuous and batch. Continuous distillation involves a constant feeding of the material to be distilled, i.e., coal tar, and the constant removal of the product or residue, i.e., coal tar pitch. Traditional continuous distillations are typically performed at pressures of between 45 mmHg and 60 mmHg and at temperatures of between 390° C. and 400° C. and are typically able to produce a coal tar pitch having a maximum softening point of approximately 140° C. Batch distillation can be thought of as taking place in a crucible, much like boiling water. High heat levels are developed as a result of the longer residence time of the coal tar in the crucible. Although higher softening points of up to 180° C. can be reached using batch distillation, the combination of high heat and longer residence time can often lead to decomposition of the coal tar pitch and the formation of unwanted mesophase pitch. Processing times for the distillation of coal tar using known continuous and batch distillation range from several minutes to several hours depending upon the coal tar pitch product to be produced.
High efficiency evaporative distillation processes are known that subject a material to elevated temperatures, generally in the range of 300° C. to 600° C., and reduced pressures generally in the range of 5 Torr or less, in a distillation vessel to evolve lower molecular weight, more volatile components from higher molecular weight, less volatile components. Such high efficiency evaporative distillation processes may be carried out using conventional distillation equipment having enhanced vacuum capabilities for operating at the above specified temperature and pressure ranges. In addition, high efficiency evaporative distillation processes may be carried out in an apparatus known as a wiped film evaporator, or WFE, and thus such processes are commonly referred to as WFE processes. Similarly, high efficiency evaporative distillation processes may be carried out in an apparatus known as a thin film evaporator, and thus such processes are commonly referred to as thin film evaporator processes. WFE and thin film evaporator processes are often used as efficient, relatively quick ways to continuously distill a material. Generally, WFE and thin film evaporator processes involve forming a thin layer of a material on a heated surface, typically the interior wall of a vessel or chamber, generally in the range of 300° C. to 600° C., while simultaneously providing a reduced pressure, generally in the range of 5 Torr or less. In a WFE process, the thin layer of material is formed by a rotor in close proximity with the interior wall of the vessel. In contrast, in a thin film evaporator process, the thin film evaporator typically has a spinner configuration such that the thin layer of material is formed on the interior wall of the vessel as a result of centrifugal force. WFE and thin film evaporator processes are continuous processes as they involve the continuous ingress of feed material and egress of output material. Both wiped film evaporators and thin film evaporators are well known in the prior art.
One prior art WFE apparatus is described in Baird, U.S. Pat. No. 4,093,479. The apparatus described in Baird includes a cylindrical processing chamber or vessel. The processing chamber is surrounded by a temperature control jacket adapted to introduce a heat exchange fluid. The processing chamber includes a feed inlet at one end and a product outlet at the opposite end.
The processing chamber of the apparatus described in Baird also includes a vapor chamber having a vapor outlet. A condenser and a vacuum means may be placed in communication with the vapor outlet to permit condensation of the generated vapor under sub-atmospheric conditions. Extending from one end of the processing chamber to the other end is a tube-like motor-driven rotor. Extending axially outward from the rotor shaft are a plurality of radial rotor blades which are non-symmetrically twisted to extend radially from one end of the chamber to the other between the feed inlet and the product outlet. The rotor blades extend into a small but generally uniform closely spaced thin-film relationship with respect to the interior wall of the processing chamber so that, when the rotor rotates, the rotor blades provide a thin, wiped or turbulent film of the processing material on the interior wall of the processing chamber.
In operation, a material to be processed is introduced into the feed inlet by a pump or by gravity. The material is permitted to move downwardly and is formed into a thin-film on the interior wall of the processing chamber by the rotating rotor blades. A heat-exchange fluid, such as steam, is introduced into the temperature control jacket so that the interior wall of the processing chamber is heated to a steady, pre-selected temperature to effect the controlled evaporation of the relatively volatile component of the processing material. A relatively non-volatile material is withdrawn from the product outlet, and the vaporized volatile material is withdrawn from the vapor chamber through the vapor outlet.